Mixtable's spreadsheet interface makes it easy to set the image you want to use for a particular product variant. You can set the variant image to an existing image of the product the variant belongs to. Or you can add a new one by uploading it.
First, ensure that you have a worksheet that has been mapped to sync with the product variants of your Shopify store. Once you have such a worksheet, we'll need to map a column to hold the variant image. Right click on the header of a column that is not yet mapped to sync to a field - such columns have a grey background, whereas columns that are set up for data syncing have a light purple background. When you right click you'll see a context menu show up. Select the Shopify Sync Settings option.
In the window that opens, select the Core Field field type, and then Variant Image from the dropdown menu. Click the Save button to save the new column mapping.
You'll see the new column start populating almost immediately with thumbnails of any images currently assigned to your variants.
Now that we have our images synced to a column we can add new ones. Each cell in the column mapped to your variant image will have a button in it labeled change. Clicking this button will present you with the Set Variant Image widget.
The Set Variant Image widget allows you to set the image for a given variant from the set of images already belonging to the same product, or by uploading a file from your computer.